With the rapid development of various industries, the consumption and discharge of flue gas, sulfur-containing industrial raw material gases and other types of exhaust gases are increasing. The discharge of sulfur-containing exhaust gases has resulted in severe environmental problems such as the formation of acid rain, corrosion of buildings by acidification, and respiratory and skin diseases, which being endangering human health. Over the years, a considerable amount of research into the desulfurization of flue gas, sulfur-containing industrial raw material gases and other types of exhaust gases have been made by scientists from around the world, and huge amounts of research data have been accumulated. With the rising awareness about environmental protection, the desulfurization of flue gas, sulfur-containing industrial raw material gases and other types of exhaust gases is being taken more seriously. Nevertheless, a breakthrough in desulfurization technology for flue gas, sulfur-containing industrial raw material gases and other types of exhaust gases has not been made so far, and the desulfurization of flue gas, sulfur-containing industrial raw material gases and other types of exhaust gases remains a challenging problem.
The current desulfurization processes can generally be classified into two main groups, namely wet processes and dry processes. Wet processes include water scrubbing, limestone and lime water process, alkali metal solution process, alkaline solution process, ammonia process, hydramine process, etc. In dry processes, SOx is removed using iron oxide, zinc oxide, manganese oxide, cobalt oxide, chromium oxide, molybdenum oxide, activated carbon, etc. In China, the most common methods are water scrubbing, limestone and lime water process, whereas developed countries prefer limestone and lime water process, alkali metal solution process, alkaline solution process, ammonia process, hydramine process, etc. In water scrubbing, a large amount of water must be consumed, and the used sulfur-containing water, which cannot be recycled, causes severe secondary pollution. Moreover, only a poor desulfurization effect can be achieved by the method of water scrubbing. Limestone and lime water process is superior to water scrubbing but is disadvantageous in that it generates a large quantity of solid waste such as calcium sulfate, calcium sulfite and calcium carbonate, consumes a large amount of limestone and calcium oxide, operates on huge equipment where blockages are apt to occur due to the formation of precipitates accompanying the absorption procedure, and requires massive investment. Besides, because of the low solubility of limestone or calcium hydroxide in water, calcium hydroxide reacts first with carbon dioxide rather than with sulfur oxides during absorption, and consequently, limestone and lime water process cannot achieve an ideal desulfurization effect and causes severe secondary pollution due to a large amount of sewage. Alkali metal solution process, alkaline solution process, ammonia process, hydramine process and the like are generally applicable to the desulfurization of, and recovery of sulfur dioxide from, flue gas with a high sulfur dioxide content, such as exhaust gas from smelting industry like steelmaking and copper smelting, in which sulfur dioxide is contained in an amount up to 8% or more. These methods, however, are technologically demanding, consume considerable energy, and require that the equipment be made of high-quality materials, so they are not suitable for the desulfurization of ordinary flue gas. Furthermore, all the processes currently used for the desulfurization of flue gas, sulfur-containing industrial raw material gases and other types of exhaust gases cause serious corrosion of the equipment.
Till now, few of the various industrial waste gases are subjected to desulfurization treatment before being discharged into the atmosphere. Even if they are desulfurized, the amount of sulfur species remaining in the discharged gases is still relatively high. Most of the current desulfurization processes (including wet processes such as HiPure process, Benfield process, G-V process, A.D.A process, water scrubbing, limestone and lime water process, alkali metal solution process, alkaline solution process, ammonia process, hydramine process, tannin extract method, sulfolane process; and dry processes such as those using iron oxide, zinc oxide, manganese oxide, cobalt oxide, chromium oxide, molybdenum oxide and activated carbon) generally serve as primary desulfurization processes to eliminate hydrogen sulfide in industrial raw material gas and are not employed to remove H2S in ordinary gas because they only achieve low desulfurization efficiency, operate at high operational costs, need massive investment for equipment, cause serious corrosion, are not ideal in desulfurization effect and cannot remove organic sulfur species in high efficiency [1-3]. Low-temperature methanol desulfurization [4] is commonly used in large chemical industry enterprises for the removal of carbon and sulfur species from raw material gas. In this method, hydrogen sulfide, carbonyl sulfide, carbon disulfide and carbon dioxide are removed by physical adsorption. However, low-temperature methanol desulfurization must work at high pressure, low temperature (as low as −10° C. or less) because of the low boiling point, volatility and high saturated vapor pressure of methanol, and thus is disadvantageous in that it consumes considerable energy, causes severe loss of methanol, is complicated in operation, and operates at high costs. In normal-temperature methanol desulfurization [5], a mixed solution of methanol (60%) and diethanolamine (40%) is used to absorb hydrogen sulfide, carbonyl sulfide, carbon disulfide and carbon dioxide from gases, and then the absorbates are released by heating and depressurizing. Because of the low boiling point, volatility and high saturated vapor pressure of methanol, a large quantity of methanol are contained in the released gas, and meanwhile the solution does not have a stable composition as a result of the serious loss of methanol. Diethanolamine is apt to oxidize and decompose when exposed to light and air, which is another cause of the instability of the solution. As a result of the above limitations, the solution is regenerated only by being heated under reduced pressure. The released sulfur-containing gas is generally converted into sulfur by the Claus method. In addition to the severe loss of methanol and diethanolamine, normal-temperature methanol desulfurization is also disadvantageous in that it consumes considerable energy, is complicated in operation, and operates at high costs. The methods discussed above are not used to get rid of SO2 and/or SO3 from gases but to remove hydrogen sulfide and organic sulfur species such as carbonyl sulfide and carbon disulfide.
Someone tried to use an aqueous solution of urotropine containing glycerin to absorb SO2 from flue gas [6]. But it was found in the actual experiment that the solution was not chemically stable due to the oxidization of urotropine by oxygen contained in the flue gas. Moreover, urotropine is a costly chemical and medical product that is not easily available. This technique has not been popularized for its high operational costs and low reliability in desulfurization effect.
A buffer of acetic acid and ammonia containing Fe2+ and Fe3+ has found application in the desulfurization of semi-water gas[7-9]. This technique is characterized by high desulfurization efficiency and low level of corrosion, but is unsatisfactory for instability of the buffer resulting from ion and salt effects. The process of catalytic decarbonization, desulfurization and decyanation of gases by means of iron-alkali solution is a wet desulfurization process capable of removing several sulfur species simultaneously, and it can achieve better effects than conventional wet desulfurization processes when used in the desulfurization of gases with low sulfur content. However, iron ions are instable in the alkali solution so as to create a large quantity of ferric hydroxide or ferrous hydroxide. Moreover, when the iron-alkali solution comes into contact with the sulfide-containing gas, a large amount of iron sulfide or ferrous sulfide is precipitated from the solution, causing a sharp decrease in the amount of iron ions in the solution, a decrease in desulfurization effect and blocking of the desulfurization tower. Thus, the iron-alkali solution process is not applicable to the desulfurization of gases with high sulfur content[10]. To improve the situation, the inventor attempted atmospheric/pressured desulfurization using an iron-alkali solution containing microorganisms and obtained good results[11]. There have been methods for the removal of hydrogen sulfide by a solution of ethylene glycol, an ethylene glycol ester, or diethylene glycol monomethyl ether. In these methods, the organic solution containing hydrogen sulfide is easily regenerated for recycle by adding thereto sulfur dioxide whereby hydrogen sulfide reacts with sulfur dioxide to produce sulfur[12-14]. These methods, however, operate at high costs under stringent safety measures because sulfur dioxide is not easily available and requires special instruments and safety measures in its transportation. To absorb hydrogen sulfide, organic sulfur species and water in natural gas or other gases, some researchers used a solution of ethylene glycol, a mixed solution of ethylene glycol and alkanolamine, a mixed solution of ethylene glycol, alkanolamine and sodium carbonate, a solution of ethylene glycol dimethyl ether or diethylene glycol dimethyl ether, a mixed aqueous solution of diethylamine, diethylene glycol, triethylene glycol and triethylene glycol monomethyl ether, a mixed solution of amine and acetaldehyde, or a mixed aqueous solution of diethylene glycol monomethyl ether and Fe(III) chelate of nitrilotriacetic acid[15-23]. The current processes discussed above are not applicable to the removal of SOx (sulfur dioxide and/or sulfur trioxide) from flue gas and other exhaust gases but are widely used for the removal of hydrogen sulfide, carbonyl sulfide and carbon disulfide from industrial raw material gas.